1. Field of the Invention
This invention relates to a solid solution particle of oxides, a process for producing the same and a catalyst for purifying exhaust gases by utilizing the solid solution particle of oxide. A solid solution of oxides of the present invention contains, for example, a solid solution of cerium oxide (called ceria hereafter) and zirconium oxide (called zirconia hereafter). In this case, an oxygen adsorption and discharge ability (an oxygen storage ability) of ceria is further improved so that the solid solution of oxides is useful as a support of a catalyst for purifying exhaust gases. Furthermore, the solid solution of oxides of the present invention can be employed as a catalyst for oxidizing a diesel particulate, a solid electrolyte, an electrode material, a reinforced particle by being dispersed in ceramics, a material for shielding ultraviolet and the like.
A catalyst for purifying exhaust gases of the present invention is excellent in its purifying activity in order to efficiently purify harmful components such as carbon monoxide, hydrocarbon, nitrogen oxide and the like contained in exhaust gases in automobiles. Even after the catalyst is subjected to high temperature, purifying performance is not remarkably degraded, and the catalyst is excellent in its durability.
2. Description of Related Arts
Ceria has been widely employed as a promotor of a catalyst for purifying exhaust gases exhausted from an internal combustion engine because it has an oxygen storage ability.
For example, a three-way catalyst oxidizes carbon monoxide and hydrocarbon contained in exhaust gases as well as it reduces nitrogen oxide. The three-way catalyst is employed in such a condition that an air/fuel (A/F) ratio is controlled to be a theoretical air/fuel ratio. However, the air/fuel ratio is deviated from the theoretical air/fuel ratio at the period of transition. In this case, if a catalyst on which ceria is loaded is employed, the air/fuel ratio can be controlled to be nearly the theoretical air/fuel ratio due to an oxygen adsorption and discharge action of ceria. As a result, the width (window) of the air/fuel ratio is widened, and purifying performance of the catalyst can be improved. Furthermore, an oxygen partial pressure is adjusted to be almost constant so that the catalyst has an effect to prevent sintering of a noble metal as a catalyst component.
In order to improve the oxygen storage ability, ceria is employed in powdery condition because a specific surface area is preferably large.
However, it is necessary that the catalyst is employed in high temperature, and that purifying activity is high after the catalyst is subjected to high temperature. So, even if ceria is employed in such a manner that it has large specific surface area as a powder, it is necessary that the large specific surface area is not damaged when ceria is employed at high temperature. Namely, ceria is required to have high heat resistance.
As a result, it has been conventionally proposed that zirconia or oxides of rare-earth elements except cerium are dissolved into ceria.
For example, Japanese Unexamined Patent Publication No. 55315/1992 discloses a process for producing a powdered body of cerium oxide in which ceria and zirconia are coprecipitated in a mixed aqueous solution comprising water soluble salt of cerium and water soluble salt of zirconium, and then, the obtained coprecipitate is subjected to heat treatment. In this process, due to such heat treatment on the coprecipitate, cerium and zirconium are changed to be oxides, and a solid solution of oxides in which zirconium is dissolved into cerium is produced.
Furthermore, Japanese Unexamined Patent Publication No. 284847/1992 discloses a process for producing a powder in which zirconia or an oxide of at least one of rare-earth elements and ceria are dissolved due to an impregnation method or a coprecipitation method.
Moreover, Japanese Unexamined Patent Publication No. 131343/1991 discloses a process for producing a catalyst for purifying exhaust gases in which a mixed oxide of ceria and zirconia is produced by utilizing a precipitated formation reaction in an aqueous solution, and then, a slurry containing this oxide is coated on a monolithic support. This catalyst for purifying exhaust gases has excellent oxygen storage ability as compared with a catalyst in which only ceria is employed or a mixed powder of ceria and zirconia is employed because zirconia is dissolved into ceria. Further, the catalyst has excellent heat resistance because of a stabilization action of zirconia.
However, even the catalyst for purifying exhaust gases produced by the process disclosed in Japanese Unexamined Patent Publication No. 131343/1991 has not still satisfied severe restrictions on exhaust gases in recent years. Furthermore, according to the research of present inventors, it is found out that the solid solution of oxides produced by the process disclosed in the above-mentioned Japanese Unexamined Patent Publication has excellent heat resistance, but the oxygen storage ability is not satisfactory.
Namely, since pH of an aqueous solution in which cerium is precipitated is different from that of an aqueous solution in which zirconium is precipitated when each of them is precipitated, the composition of a coprecipitate is not uniform in the coprecipitation method in which the coprecipitate is coprecipitated in the mixed aqueous solution. As a result, a dissolution is hardly occurred at the stage of coprecipitate. Further, in the impregnation method, there are two cases: (1) a powder of ceria is impregnated in an aqueous solution containing salt of zirconium and (2) a powder of zirconia is impregnated in an aqueous solution containing salt of cerium. In either case, the composition of whole particles is not uniform because a primary particle is large. As a result, a dissolution is hardly promoted
Therefore, in both methods, the dissolution is occurred by heat treatment. The heat treatment is performed at the temperature of 500-900.degree. C. in the coprecipitation method, and 700-1200.degree. C. in the impregnation method. Though, the dissolution is not complete. Furthermore, in both methods, a neck is formed between particles due to heat treatment on the powder, and the dissolution is promoted by the neck. In accordance with this, a grain growth and sintering are promoted. So, a specific surface area of the powder is decreased, and a particle diameter of a crystallite is increased. Moreover, once solid solution particle of oxides grow large, they cannot be easily pulverized.
For example, the degree of dissolution of the solid solution of oxides obtained by the process disclosed in Japanese Unexamined Patent Publication No. 55315/1992 is at most 40%. Furthermore, the degree of dissolution of the solid solution of oxides obtained by the process disclosed in Japanese Unexamined Patent Publication No. 284847/1992 is not more than 20%. Thus, in such conventional solid solution of oxides comprising ceria and zirconia having low degree of dissolution, an oxygen storage capacity (called OSC hereafter) of ceria is small, namely, at most 100-150 .mu.molO.sub.2 /g. Furthermore, if the temperature is not more than 500.degree. C., the oxygen adsorption and discharge ability is not satisfactorily exhibited, and the oxygen storage ability is poor.
It is known that the degree of dissolution of the solid solution of oxides comprising ceria and zirconia is almost 100% when the heat treatment is fully performed at the temperature of approximately 1600.degree. C. However, in this case, although OSC is large, an average diameter of a crystallite is not less than 1000 nm. As a result, the specific surface area is small, and an oxygen adsorption and discharge speed at the period of transition is regulated by the small specific surface area. Therefore, it has no practical use as a promotor.